Organic light-emitting devices, also referred to as organic electroluminescent (EL) devices or as organic internal junction light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits electromagnetic radiation, typically light, in response to the application of an electrical potential difference across the electrodes. At least one of the electrodes is light transmissive, and the organic light-emitting structure can have from 2-4 overlying organic thin film layers which provide for hole injection and transport from an anode, and for electron injection and transport from a cathode, respectively, with light emission resulting from electron-hole recombination at an internal junction formed at an interface between the hole-transporting and the electron-transporting thin films. As employed herein, the term "thin film" refers to layer thicknesses of less than 5 micrometer with layer thickness of less than about 2 micrometer being typical. Examples of organic light-emitting devices containing organic light-emitting structures and cathode constructions formed by thin film deposition techniques are provided by Tang U.S. Pat. No. 4,356,429; VanSlyke et al., U.S. Pat. Nos. 4,539,507 and 4,720,432; and Tang et al., U.S. Pat. No. 4,769,292.
During operation of an organic light-emitting device, the spectral distribution of emitted light (measured in terms of spectral radiance) is related to the electroluminescent properties of the organic thin films used in the device construction. For example, if an organic light-emitting structure includes a light-emitting layer which contains a light-emitting host material, the emitted light will be reflective of, or be dominated by, the host light emission from the host material.
Tang et al., in the above-cited U.S. Pat. No. 4,769,292, recognized that advantageous performance features of an organic light-emitting device could be obtained if the device included a luminescent zone (or light-emitting layer) of less than 1 micrometer in thickness and comprised of an organic host material capable of sustaining hole-electron recombination, and a fluorescent material capable of emitting light in response to energy released by hole-electron recombination. The introduction of a fluorescent material into a layer of a light-emitting host material will modify the host light emission, and can improve the operational stability of an organic light-emitting device. In analogy to terminology used in the semiconductor industry, fluorescent materials dispersed uniformly at relatively low concentration in light-emitting organic host materials are called "dopants."
Such organic light-emitting devices which emit light in response to an applied voltage, and cease to emit light when the applied voltage is removed, are constructed with an anode and a cathode which are each unitary elements whose light emission can be turned on or turned off, but which lack an image display capability when used alone.
When an organic EL device is to receive an image display capability, the device is constructed with a plurality of light-emitting pixels which are generally arranged as an array in intersecting columns and rows. This is achieved by patterning the anode and cathode of the former unitary elements into a plurality of individually electrically addressable anode and cathode electrodes which are orthogonally oriented with respect to each other, with the organic EL medium disposed between the plurality of anodes and the plurality of intersecting cathodes. Such an organic EL display, also referred to variously as an image display or an image display panel, has an organic EL medium which uniformly (i.e., unpatterned) extends laterally between the respective sets of anode and cathode electrodes, and is capable of monochrome image display, that is a display providing emission of light having a single hue.
In constructing a full-color organic light-emitting display, at least two additional requirements have to be met compared to fabricating a monochrome organic light-emitting display:
(1) each one of the plurality of anodes or each one of the plurality of cathodes of the monochrome display is further partitioned to provide sets of three laterally spaced subpixels; and PA1 (2) each subpixel of a set of subpixels must be capable of emitting one of red, green, and blue light, respectively, so that an observer of the full-color organic light-emitting display perceives the full-color capability similar to that perceived in watching a full-color television display or a full-color liquid crystal display PA1 Full-color organic light-emitting displays meeting the aforementioned requirements can be grouped into two basic designs and corresponding methods of making the displays: PA1 a) providing a substrate; PA1 b) providing a first set of addressing electrodes over the substrate; PA1 c) forming an organic hole-transporting layer over the first set of addressing electrodes and over the substrate; PA1 d) forming an organic light-emitting layer over the hole-transporting layer and having a light-emitting material selected to produce blue light; PA1 e) forming a dopant layer over the light-emitting layer and patterning such dopant layer to form color subpixels which comprise a pixel wherein the colored subpixels are formed by diffusing the patterned dopant layer into the light-emitting layer; PA1 f) forming an organic electron-transporting layer over the doped light-emitting layer; and PA1 g) forming a second set of addressing electrodes over the electron-transporting layer so that the color subpixels can be individually addressed.
(i) a display having the subpixel pattern of the cathodes or the subpixel pattern of the anodes is fabricated as a monochrome light-emitting display having a light emission in a blue spectral region, i.e., each subpixel is capable of emitting blue light when addressed by an electrical drive signal; and a color conversion medium oriented with respect to the subpixels and capable of converting blue light emitted from a corresponding subpixel into one of red and green light, and capable of transmitting the blue light emitted by a corresponding subpixel, respectively; designs of this grouping are exemplified by disclosures of Tang et al. in U.S. Pat. No. 5,294,870, with a particularly preferred form of such a display described with reference to FIGS. 4, 5, and 6 thereof; PA2 (ii) the organic EL medium, or at least an organic light-emitting layer thereof, is patterned directly in forming the display and in correspondence with the subpixel pattern of the electrodes; and the organic light-emitting layer within each subpixel is formed of an organic light-emitting host material and a fluorescent dopant selected to provide one of red, green, and blue light emission directly from a corresponding subpixel; exemplary of this second group of full-color organic light-emitting displays is an image display device disclosed by Tang et al. in U.S. Pat. No. 5,294,869, with particularly preferred embodiments described with reference to FIGS. 15, 17, and 18 thereof.
A feature of a display device fabricated in accordance with the first design group of devices is that each subpixel is formed of one and the same blue light-emitting organic EL medium. A principal disadvantage is that the color conversion medium of this device is spaced in a vertical direction from the EL medium of each of the corresponding subpixels, and a potential reduction of luminous efficiency due to a conversion efficiency which is less than 100 percent for converting the blue light into green and red light, respectively.
A feature of an image display device constructed in accordance with the second group of devices is a potentially enhanced luminous efficiency (since no conversion is required). In order to more fully appreciate one impediment to reliably and reproducibly fabricate such a display, the current method of fabrication will be briefly reviewed as follows.
The organic thin films of the light-emitting display are formed by vapor phase deposition (evaporation or sublimation) in successive deposition steps within a vacuum system which employs a deposition rate control. When a fluorescent dopant is to be uniformly incorporated within an organic light-emitting layer of a light-emitting host material, the light-emitting host material and the fluorescent dopant material are co-deposited from two independently controlled deposition sources. Thus, for example, the light-emitting host material and a fluorescent dopant material selected to provide red light emission from a designated subpixel, are co-deposited from two independently controlled deposition sources. Similarly, a subpixel designated for green light emission has its doped light-emitting layer formed by co-deposition of a host material and a fluorescent dopant material capable of providing green light emission. The same deposition process is followed for the subpixel designated to emit blue light or spectrally modified blue light.
It has been found to be difficult to reliably control the deposition rate of a fluorescent dopant when a desired dopant concentration in the host material of an organic light-emitting layer is at or near a lower end of a dopant concentration range of about 10.sup.-3 to about 10 mole percent. The difficulty of reliably controlling the deposition rates of an organic light-emitting host material and of a fluorescent dopant material has been an impediment in the process of reproducibly fabricating full-color organic light-emitting displays, particularly when more than one fluorescent dopant is to be incorporated in the light-emitting layer of a subpixel so as to tailor the hue of the red, green, or blue light emitted from a corresponding subpixel.